This invention relates generally to treatment of selected tissue by inter-vivo radiation, specifically to radiation treatment of selected regions of the cardiovascular system that have been subjected to trauma to prevent restenosis of the traumatized region, more specifically to radiation treatment to prevent restenosis of an artery traumatized by percutaneous transluminal angioplasty (PTA).
PTA treatment of the coronary arteries, percutaneous transluminal coronary angioplasty (PTCA), also known as balloon angioplasty, is the predominant treatment for coronary vessel stenosis. Approximately 300,000 procedures were performed in the United States (U.S.) in 1990 and an estimated 400,000 in 1992. The U.S. market constitutes roughly half of the total market for this procedure. The increasing popularity of the PTCA procedure is attributable to its relatively high success rate, and its minimal invasiveness compared with coronary by-pass surgery. Patients treated by PTCA, however, suffer from a high incidence of restenosis, with about 35% of all patients requiring repeat PTCA procedures or by-pass surgery, with attendant high cost and added patient risk. More recent attempts to prevent restenosis by use of drugs, mechanical devices, and other experimental procedures have had limited success.
Restenosis occurs as a result of injury to the arterial wall during the lumen opening angioplasty procedure. In some patients, the injury initiates a repair response that is characterized by hyperplastic growth of the vascular smooth muscle cells in the region traumatized by the angioplasty. The hyperplasia of smooth muscle cells narrows the lumen that was opened by the angioplasty, thereby necessitating a repeat PTCA or other procedure to alleviate the restenosis.
Preliminary studies indicate that intravascular radiotherapy (IRT) has promise in the prevention or long-term control of restenosis following angioplasty. It is also speculated that IRT may be used to prevent stenosis following cardiovascular graft procedures or other trauma to the vessel wall. Proper control of the radiation dosage, however, is critical to impair or arrest hyperplasia without causing excessive damage to healthy tissue. Overdosing of a section of blood vessel can cause arterial necrosis, inflammation and hemorrhaging. Underdosing will result in no inhibition of smooth muscle cell hyperplasia, or even exacerbation of the hyperplasia and resulting restenosis.
U.S. Pat. No. 5,059,166 to Fischell discloses an IRT method that relies on a radioactive stent that is permanently implanted in the blood vessel after completion of the lumen opening procedure. Close control of the radiation dose delivered to the patient by means of a permanently implanted stent is difficult to maintain because the dose is entirely determined by the activity of the stent at the particular time it is implanted. Additionally, the dose delivered to the blood vessel is non-uniform because the tissue that is in contact with the individual strands of the stent receive a higher dosage than the tissue between the individual strands. This non-uniform dose distribution is especially critical if the stent incorporates a low penetration source such as a beta emitter.
U.S. Pat. No. 5,302,168 to Hess teaches use of a radioactive source contained in a flexible carrier with remotely manipulated windows. H. Bottcher, et al. of the Johann Wolfgang Goerhe University Medical Center, Frankfurt, Germany report in November 1992 of having treated human superficial femoral arteries with a similar endoluminal radiation source. These methods generally require use of a higher activity source than the radioactive stent to deliver an effective dose. Accordingly, measures must be taken to ensure that the source is maintained reasonably near the center of the lumen to prevent localized overexposure of tissue to the radiation source. Use of these higher activity sources also dictates use of expensive shielding and other equipment for safe handling of the source.
What is needed theta, is an IRT method and apparatus that delivers an easily controllable uniform dosage of radiation to the walls of the blood vessel without the need for special measures to center a radiation source in the lumen, the need for expensive shielding to protect medical personnel, or the need for expensive remote afterloaders to handle the higher activity sources.